The Posidonia Shale around Holzmaden, about 180 million years old, was formed by favorable depositional conditions of a tropical shallow sea once located there from the Jurassic period.
There, the best preserved remains of the marine life of that time can be found, such as ammonites, belemnites and shells. 
Also finds like fossil driftwood (gagat), fish, bones of marine reptiles, like the world-famous Ichthyosaurus or even single remains of pterosaurs are possible.